Conventionally, there is known a welding detection method for the electromagnetic relay in which a movable iron core is fixed to a driving shaft that is reciprocally provided in a center axis direction of an excitation coil and the movable contact fixed to the driving shaft is attracted to or separated from a fixed contact by an electromagnetic force generated in the movable iron core in accordance with supply of cutoff of a current passed through the excitation coil (Patent Documents 1 and 2).
In the electromagnetic relay, the attraction of the movable contact to the fixed contact differs from the separation of the movable contact from the fixed contact in an inductance of the excitation coil because a position of the movable iron core relative to the excitation coil changes. Therefore, there is known a configuration that detects the inductance of the excitation coil to detect welding between the movable contact and the fixed contact.
For example, Patent Document 2 discloses the following configuration. A switching element connected to an operating coil (excitation coil) performs on and off operations using a pulse signal. An AC component is extracted by cutting a DC component and a low-frequency component from a coil current of the operating coil. Opening and closing malfunctions of the electromagnetic relay are detected on the basis of the AC component of the coil current and a criterion.
Patent Document 1 discloses the following configuration in addition to the configuration of Patent Document 2. A step voltage that switches between a high level and a low level or a pulse voltage having a sufficiently low frequency is periodically applied to the excitation coil. A rising time is detected until a transient response component of a voltage at both ends of a detection coil exceeds a predetermined reference value since the application is started. The detection of the rising time discriminates between the opening and closing of a contact part (movable contact) to detect opening and closing states of the electromagnetic relay.